The goal of the proposed research is to understand the cellular, genetic, and molecular mechanisms of chemosensory response to the dauer-inducing pheromone of Caenorhabditis elegans. The dauer larva is a developmentally distinct alternative third-larval stage (L3) that is specialized for long- term survival under harsh conditions. The choice between the normal L3 and the dauer larva is controlled primarily by chemosensory assessment of the environmental concentration of a secreted dauer-inducing pheromone. More than 25 genes have been identified that control the process of dauer formation and they have been placed into a complex epistasis pathway. Some of these genes are implicated in the chemosensory process per se, while others probably act downstream of chemosensation to activate the dauer- larval developmental program. This proposal concentrates on the genes implicated in the function and development of the dauer pheromone chemosensory cells. One aim is to rigorously test the role of sensory neurons that have been implicated in controlling pheromone response. Pheromone responsiveness will be measured in animals in which individual identified neurons have been eliminated with a laser microbeam. Another aim is to complete a thorough search for mutations in genes important for the function and development of these sensory cells. Mutations in genes already implicated in pheromone chemosensation will be used to identify other such genes using a variety of classical genetic approaches. One gene, daf-11, already implicated specifically in the process of pheromone chemosensation, will be cloned by transposon tagging. A variety of molecular biological and genetic approaches will be taken to defining the function of daf-11, including DNA sequence analysis, gene product localization, and genetic mosaic analysis. Another gene, daf-19, is implicated in the development of a large set of sensory neurons, including those controlling dauer formation. daf-19 will be cloned using detailed genetic and physical mapping and DNA transformation. The cloned gene will be used to test the involvement of daf-19 in the development of the sensory cells by DNA sequence analysis and expression studies. Sensory modulation of dauer formation in C. elegans provides an excellent model for the investigation of sensory and environmental influences on development. The opportunity to both apply extensive genetic analysis to such a problem and to define the sensory pathway at the single neuron level is unique. In addition, response to dauer pheromone is a model for the general process of chemosensation, which is currently poorly understood.